Conventionally, in a boiling water reactor, a so-called jet pump system obtained by combining a recirculation pump installed outside a reactor pressure vessel and a jet pump installed inside the reactor pressure vessel is for the purpose of increasing an output power density.
As illustrated in FIG. 22, a plurality of jet pumps 4 are circumferentially arranged at equal intervals in a downcomer portion 3 between a reactor pressure vessel 1 and a shroud 2 each having an axis extending in a vertical direction. As illustrated in FIG. 23 which illustrates a main part of FIG. 22 in an enlarged manner, the jet pump 4 has a riser pipe 5. The riser pipe 5 is fixed to the reactor pressure vessel 1 and introduces coolant supplied from a recirculation inlet nozzle 6 of a recirculation pump into the reactor.
A pair of elbows 7A and 7B are connected to an upper portion of the riser pipe 5 through a transition piece 14. The pair of elbows 7A and 7B are connected with a pair of inlet throats 9A and 9B, respectively, through a pair of mixing nozzles 8A and 8B, respectively. The pair of inlet throats 9A and 9B are connected with diffusers 10A and 10B, respectively.
Hereinafter, the pair of inlet throats 9A and 9B and the pair of diffusers 10A and 10B are sometimes interchangeably or collectively referred to as inlet throat 9 and diffuser 10, respectively.
It is important to measure a flow rate of the jet pump 4 in a normal operation state in order to perform output power control of a nuclear power plant. Thus, a measuring pipe 11 is provided so as to extend from upper to lower portions of the diffusers 10A and 10B, respectively. Using the measuring pipe 11 to measure a difference in static pressure between the upper and lower portions of the diffuser 10, and a measurement value is calibrated by a calibration value obtained before use of a plant, whereby the flow rate of the jet pump 4 is calculated.
The measuring pipe 11 is welded to static pressure holes of the upper and lower portions of the diffuser 10 and welded to/supported by support members, such as a block 12 and a support 13 (FIG. 24), fixed to the diffuser 10. Further, as illustrated in FIGS. 25A and 25B, the measuring pipe 11 is disposed in a complicated manner at the lower portions of the jet pumps 4 and is connected to piping outside the reactor through a jet pump measuring nozzle 15. The jet pump measuring nozzle 15 is provided at two symmetrical positions in a horizontal cross-section of the reactor pressure vessel 1.
The jet pump 4 having the above configuration is used under severe conditions than those for other devices due to high temperature of about 300 degrees centigrade and flow of high speed and large flow coolant water fed by means of a not illustrated recirculation pump. Thus, a large load is applied to each member and, particularly, the measuring pipe 11 subjects to fluid vibration caused by the flow of high speed and large flow coolant water fed by means of the recirculation pump of the diffuser 10 directly or through the block 12 or the support 13, with the result that severe stress is applied to the measuring pipe 11. This may cause a rupture in the measuring pipe 11. If the measuring pipe 11 is ruptured, a flow rate of the jet pump 4 cannot be measured, affecting output power control of the reactor. Thus, repair work is urgently required.
As illustrated in FIG. 25B, the measuring pipe 11 is disposed in a narrow annular space 16 between the reactor pressure vessel 1 and the shroud 2, and the riser pipe 5, the inlet throat 9, and the like are disposed above the measuring pipe 11, as illustrated in FIG. 23. A horizontal part of the measuring pipe 11 near the support 13 illustrated in FIG. 24 is positioned closest to the shroud 2, and an interval between the horizontal part and the shroud 2 is less than 150 mm.
Further, an upper portion of the horizontal part of the measuring pipe 11 is overhung with an intermediate body of the shroud 2. This limits a shape and a size of a repair device to the measuring pipe 11 and a repair method, making the repair work very difficult.
Further, a site around which the measuring pipe 11 is installed is a high-radiation area, making it very difficult for a worker to get close to the piping section. Thus, under present circumstances, there is no way other than to perform the repair work of the measuring pipe 11 underwater in a remote-controlled manner from just above a core.
As a repair method for such a measuring pipe 11, there are methods using a welding device conducting welding after discharge of reactor water and an underwater laser welding device (refer to, e.g., Patent Documents 1 and 2).